The present invention generally relates to contact type image sensors, and more particularly to a contact type image sensor for use in a document scanner and the like.
In a document scanner, the overall size of the document scanner inevitably becomes large when an image sensor requires an optical reduction system. Hence, there are document scanners which use a contact type image sensor having a size in correspondence with a maximum document size which is to be read. In other words, the contact type image sensor extends for a length corresponding to a maximum width or length of the document which is to be read, and there is no need to use an optical reduction system.
FIG. 1 shows a circuit construction of an example of a conventional contact type image sensor. A contact type image sensor 10 has a photodetector 2 made up of a plurality of photoelectric conversion elements 1, an analog switch part 3 made up of a plurality of switches, a buffer part 4 made up of a plurality of buffers, a driving circuit 5 and a signal processing circuit 9. First terminals of the photoelectric conversion elements 1 are connected to the corresponding switches of the analog switch part 3.
Each photoelectric conversion element 1 is shown as having a photodiode structure with a photodiode PD and a sensor capacitance CD connected in parallel. A switch of the switch part 3 and a buffer of the buffer part 4 are provided in correspondence with each photoelectric conversion element 1. The driving circuit 5 is constituted by a shift register and sequentially operates the switches of the analog switch part 3 responsive to clock signals .phi..sub.CLK and .phi..sub.CLK and a start pulse signal SP so as to selectively drive the photoelectric conversion elements 1. Each bit of the photodetector 2, that is, each photoelectric conversion element 1 corresponds to one bit of the shift register (5). Similarly, each switch of the analog switch part 3 corresponds to one bit of the shift register (5). MP denotes a power source voltage for the photodetector 2.
The signal processing circuit 9 is connected in common to second terminals of the photoelectric conversion elements 1. The signal processing circuit 9 has a current-to-voltage (C/V) converting circuit 6, a peak holding circuit 7 and an analog-to-digital (A/D) converter 8. For example, the C/V converting circuit 6 has operational amplifiers coupled in two stages. An output signal of the C/V converting circuit 6 is passed through the peak holding circuit 7 and the A/D converter 8, and an output digital data is obtained from the A/D converter 8. For example, the A/D converter 8 has an integrating circuit.
When reading a document image, all of the bits of the photodetector 2 are set to a high level and all of the sensor capacitances CD are charged. A light is irradiated on each bit of the photodetector 2 depending on black and white portions of the document image, and the sensor capacitances CD of those photoelectric conversion elements 1 which receive the light discharge. The driving circuit 5 sequentially turns ON the switches of the analog switch part 3 and a resulting current which flows through the photodiode PD is converted into a voltage in the C/V converting circuit 6. The peak holding circuit 7 reads a voltage dependent on the intensity of the light by holding the peak value of the output voltage of the C/V converting circuit 6. The A/D converter 8 converts the read voltage from the peak holding circuit 7 into a digital data describing gradation levels of the document image.
When the start pulse signal SP is applied to the driving circuit (shift register) 5, an output signal SO of the signal processing circuit 9 shown in FIG. 2(L) is sequentially obtained in synchronism with the clock signals .phi..sub.CLK and .phi..sub.CLK shown in FIGS. 2(A) and 2(B) because bit numbers "1" through "5" and "1725" through "1728" shown in FIGS. 2(C) through 2(K) are respectively outputted from the driving circuit 5. FIGS. 3(A) and 3(B) respectively show an output signal CVO of the C/V converting circuit 6 and an output signal PHO of the peak holding circuit 7. In other words, a voltage in correspondence with the intensity of the light is read by the peak holding circuit 7, and the output signal PHO of the peak holding circuit 7 is converted into the digital data (sensor output) SO describing the gradation levels of the document image.
FIG. 4 shows a perspective view of the conventional contact type image sensor 10. The photoelectric conversion elements 1 are arranged in a line on a transparent dielectric substrate 11 such as a silicon substrate so as to constitute the photodetector 2. The analog switch part 3, the buffer part 4 and the driving circuit 5 are formed on the substrate 11 to constitute a driving circuit part 12.
Each of the photoelectric conversion elements 1 have a sandwich structure shown in FIG. 5. A protection layer 13, an insulator layer 14 and a metal electrode layer 15 are successively formed on the substrate 11, and a semiconductor layer 16, a protection layer 17 and a transparent electrode layer 18 are further provided as shown. For example, the semiconductor layer 16 is made of amorphous Si including H or amorphous Si including OH.
On the other hand, the driving circuit part 12 has a metal oxide semiconductor (MOS) inverter structure using polysilicon for an activation layer (planar thin film transistor structure) as shown in FIGS. 6A and 6B, where FIG. 6A shows a plan view and FIG. 6B shows a cross sectional view of the driving circuit part 12. An activation layer 20 is patterned on the substrate 11, and a p.sup.+ -type diffusion layer 21 is formed on both sides of the activation layer 20. A gate oxide layer 22 is formed on the activation layer 20, and a gate electrode 23 is formed on the gate oxide layer 22. An insulator layer 24 covers the exposed gate oxide layer 22 and the like on the substrate 11. A contact hole 25 is formed in a portion of the insulator layer 24, and a metal electrode 26 is formed on the insulator layer 24 to electrically connect to the p.sup.+ -type diffusion layer 21 through the contact hole 25. Hence, the analog switch part 3 has a p-type MOS structure. The driving circuit 5 has an n-type MOS structure and is located adjacent to the analog switch part 3. The driving circuit 5 basically uses the same structure as the analog switch part 3 except that a portion corresponding to the p.sup.+ -type diffusion layer 21 is an n.sup.+ -type diffusion layer 27 in the driving circuit 5. The surface of the driving circuit part 12 is covered by a protection layer (not shown).
Therefore, the driving circuit 5 and the like of the contact type image sensor 10 are integrally formed on the transparent substrate 11 by use of thin film transistors or the like. However, in order to drive one photoelectric conversion element 1, that is, with respect to one bit of the photodetector 2, it is necessary to provide twenty or more thin film transistors. As a result, in the case of the contact type image sensor designed to read a document of A4 size, for example, it is necessary to provide a large number of photoelectric conversion elements 1 amounting to 1728 bits (=8 bits/mm.times.216 mm), and 34560 (=1728 bits.times.20) or more transistors are required to drive such a large number of photoelectric conversion elements 1.
But it is extremely difficult to form such a large number of transistors on the substrate 11 in a satisfactory state because it is difficult to form a perfectly uniform thin film on such a large area of the substrate 11 that is sufficient to provide the large number of transistors. For this reason, there is a problem in that the yield of the conventional contact type image sensor is poor. In addition, even when a single transistor of the driving circuit 5 fails, there is a problem in that the driving circuit 5 thereafter carries out an erroneous operation or the driving circuit 5 does not operate at all.